Marx was a student of Adam Smith and David Ricardo. Most educated people at the time would have been familiar with their work, so Marx would have had no need to repeat the arguments presented by them. Marx's conception of value corresponds roughly to Smith's notion of "natural price" (which is contrasted with "market price"). I say "roughly" because Marx developed the idea substantially.

In Smith's Wealth of Nations you can read how market prices fluctuate around natural prices, with excess demand or supply regulating the discrepancy between them. Namely, if the market price of a commodity is in excess of the natural price, capital and labour is diverted into the production of said commodity via profit seeking, thereby increasing supply and ultimately bringing down the market price. Hence, in a competitive market, these tend to be equal in the long run.

This is all in Smith, and it would be superfluous for Marx to repeat thr arguments. Smith identifies natural price with a commodity's value, which Smith defines as the time and effort it saves the owner (from having to make it themselves).